1. Field Of The Invention
This invention relates to amorphous metallic transformer cores having increased operating induction; and more particularly, to a magnetic field annealing process that markedly increases such operating induction.
2. Description Of The Prior Art
Soft magnetic properties of amorphous metallic transformer core alloys are developed as a result of annealing at suitable temperature and time in the presence of a magnetic field. One of the purposes for such annealing is to reduce the adverse effects of residual stresses which result from the rapid cooling rate associated with amorphous alloy manufacturing processes. Another purpose is to define the "magnetic easy axis" in the body being annealed; i.e. to define a preferred orientation of magnetization which would ensure low core loss and exciting power of the body being annealed. Historically, such magnetic field annealing has been performed to minimize the core loss of the annealed body, as disclosed U.S. Pat. Nos. 4,116,728 and 4,528,481 for example. In addition to magnetic field annealing, annealing of amorphous alloys while under tensile stress has also been shown to result in improved soft magnetic properties viz. U.S. Pat. Nos. 4,053,331 and 4,053,332. Sample configuration for tensile stress annealing has invariably been flat strip. The use of stress annealing in the production on amorphous alloy transformers is impracticable.
The two most important magnetic properties of a transformer core are the core loss and exciting power of the core material. When magnetic cores of annealed metallic glass are energized (i.e., magnetized by the application of a magnetic field) a certain amount of the input energy is consumed by the core and is lost irrevocably as heat. This energy consumption is caused primarily by the energy required to align all the magnetic domains in the amorphous metallic alloy in the direction of the field. This lost energy is referred to as core loss, and is represented quantitatively as the area circumscribed by the B-H loop generated during one complete magnetization cycle of the material. The core loss is ordinarily reported in units of W/kg, which actually represents the energy lost in one second by a kilogram of material under the reported conditions of frequency, core induction level and temperature.
Core loss is affected by the annealing history of the amorphous metallic alloy. Put simply, core loss depends upon whether the alloy is under-annealed, optimally annealed or over-annealed. Under-annealed alloys have residual, quenched-in stresses and related magnetic anisotropy's which require additional energy during magnetization of the product and result in increased core losses during magnetic cycling. Over-annealed alloys are believed to exhibit maximum atomic "packing" and/or can contain crystalline phases, the result of which is a loss of ductility and/or inferior magnetic properties such as increased core loss caused by increased resistance to movement of the magnetic domains. Optimally annealed alloys exhibit a fine balance between ductility and magnetic properties. Presently, transformer manufacturers utilize annealing conditions which minimize the core loss of the amorphous metallic alloy transformer core. Typically, core loss values of less than 0.37 W/kg (60 Hz and 1.4 T) are achieved.
Exciting power is the electrical energy required to produce a magnetic field of sufficient strength to achieve in the metallic glass a given level of induction (B) . Exciting power is proportional to the required magnetic field (H), and hence, to the electric current in the primary coil. An as-cast iron-rich amorphous metallic alloy exhibits a B-H loop which is somewhat sheared over. During annealing, as-cast anisotropies and cast-in stresses are relieved, the B-H loop becomes more square and narrower relative to the as-cast loop shape until it is optimally annealed. Upon over-annealing, the B-H loop tends to broaden as a result of reduced tolerance to strain and, depending upon the degree of over-annealing, existence of crystalline phases. Thus, as the annealing process for a given alloy progresses from under-annealed to over-annealed, the value of the exciting power for a given level of magnetization initially decreases, then reaches an optimum (lowest) value, and thereafter increases. However, the annealing conditions which produce an optimum (lowest) value of exciting power in an amorphous metallic alloy do not coincide with the conditions which result in lowest core loss. As a result, amorphous metallic alloys, annealed to minimize core loss do not exhibit optimal exciting power.
It should be apparent that optimum annealing conditions are different for amorphous alloys of different compositions, and for each property required. Consequently, an "optimum" anneal is generally recognized as that annealing process which produces the best balance between the combination of characteristics necessary for a given application. In the case of transformer core manufacture, the manufacturer determines a specific temperature and time for annealing which are "optimum" for the alloy employed and does not deviate from that temperature or time.
In practice, however, annealing ovens and oven control equipment are not precise enough to maintain exactly the optimum annealing conditions selected. In addition, because of the size of the cores (typically up to 200 kg each) and the configuration of ovens, cores may not heat uniformly, thus producing over-annealed and under-annealed core portions. Therefore, it is of utmost importance not only to provide an alloy which exhibits the best combination of properties under optimum conditions, but also to provide an alloy which exhibits that "best combination" over a range of annealing conditions. The range of annealing conditions under which a useful product can be produced is referred to as an "annealing (or anneal) window".